panic: sandbox escaping file name "../file0"

goroutine 16 [running]:
github.com/google/syzkaller/prog.(*randGen).filename(0xc00057d400, 0xc0004c3f40, 0xbf4ce0, 0x0, 0x0)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/rand.go:161 +0x2ac
github.com/google/syzkaller/prog.(*BufferType).mutate(0xbf4ce0, 0xc00057d400, 0xc0004c3f40, 0x8fa9e0, 0xc0006b6bd0, 0xc0004c3f08, 0xc0006b68a0, 0x0, 0xc0006b6c00, 0x1161bffb8f50af0a, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:264 +0x533
github.com/google/syzkaller/prog.(*Target).mutateArg(0xc0000731e0, 0xc00057d400, 0xc0004c3f40, 0x8fa9e0, 0xc0006b6bd0, 0xc0004c3f08, 0xc0006b68a0, 0x0, 0x4f0f00, 0xc0007d7e0f, ...)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:172 +0xe3
github.com/google/syzkaller/prog.(*mutator).mutateArg(0xc0007d7ed0, 0xa)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:153 +0x231
github.com/google/syzkaller/prog.(*Prog).Mutate(0xc0004c3ec0, 0x8f8660, 0xc000691410, 0x1e, 0xc0006d5940, 0xc000648000, 0x843, 0x900)
	/syzkaller/gopath/src/github.com/google/syzkaller/prog/mutation.go:34 +0x2db
main.(*Proc).loop(0xc0006d5980)
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/proc.go:99 +0x446created by main.main
	/syzkaller/gopath/src/github.com/google/syzkaller/syz-fuzzer/fuzzer.go:236 +0xfe2